Serogroup B Neisseria meningitidis infections are a major cause of morbidity and mortality worldwide. Complement (C) is important in combating neisserial infections. Individuals with C deficiency are predisposed to neisserial infections. Antibodies (Abs) that induce bactericidal killing protect against invasive disease. We have identified lipooligosaccharide (LOS) as a major C4b target when other than specific anti-capsular Abs are used for opsonization. Phosphoethanolamine (PEA) on heptose 2 (Hep2) of LOS is an important acceptor for C4 (based on our data that C4b binds LOS via amide bonds). In specific Aim 1 we will study the relative roles of PEA residues in the 3- and 6-position of Hep2 in accepting C4b, by comparing C4b binding to LOS on strains that differ only in the location of PEA (either the 3- or 6-position) on Hep2. We have determined that Hep1 chain substitutions influence the nature of the linkage (ester or amide) formed between C4b and LOS. We will extend these findings and study the effect of Hep2 hexose substitutions (the IgtG gene product) on C4b binding to LOS. In Specific Aim 2, we will examine the ability of the isoforms of C4 in human serum to bind to LOS in the context of whole bacteria. Two isoforms of C4 with differing biological activities, called C4A and C4B, exist in human serum. While C4B is ~3 times more hemolytically active, C4A engages its target via exclusively amide linkages and binds complement receptor 1 (CR1) more efficiently. The ability of bacteria opsonized with serum selectively deficient in C4A and C4B to bind to CR1 will be studied in a chinese hamster ovary (CHO) cell line transfected with a mutant CR1 molecule that contains only the C4b binding site. Finally, in specific Aim 3, we will examine whether bacterial targets for C4b binding are altered by mAbs against a repertoire of bacterial antigens, in an effort to understand why certain antigenic targets (such as class 4 protein) are not good bactericidal targets despite being recognized by C-fixing Abs. We will also determine the relative efficiency with which two mAbs that deposit C4b on to different targets elicit C-mediated killing, in an attempt to define the requirements for a vaccine candidate to elicit the most efficient bactericidal (or opsonic) response. [unreadable] [unreadable] [unreadable]